Metal-air cathode can having reduced corner radius and electrochemical cells made therewith

ABSTRACT

This invention pertains to electrode cans and metal air electrochemical cells made with the electrode cans. The invention provides improved structure, and methods for making the outer edge of the closed end of the can at the joinder of the closed end of the can with an side wall extending from the closed end. A substantially flat portion of the outer surface of the closed end of the can extends outwardly of the inner surface of the side wall. The electrochemical cells are assembled using improved assembly methods. Button-type electrochemical cells made using the invention are free of the inward dishing common to especially cathode cans in such button cells.

This application is a continuation of application Ser. No. 08/435,770,filed May 5, 1995.

FIELD OF THE INVENTION

This invention relates to alkaline electrochemical cells having metallicanodes and air cathodes, commonly known as metal-air cells. Moreparticularly, this invention relates to the structure of the cathode canand the anode can, and to the methods and apparatus used to form thecans, and to assemble the cans to each other in making anelectrochemical cells.

BACKGROUND OF THE INVENTION

The growth in use of small electrically-powered devices has increasedthe demand for very small metal-air electrochemical cells. Such smallcells are usually disc-like or pellet-like in appearance, and are aboutthe size of garment buttons. These cells generally have diametersranging from less than 0.25 inch up to about 1.0 inch, and heightranging from less than 0.15 inch up to about 0.60 inch. The small size,and the limited amount of electrochemically reactive material which canbe contained in these small metal-air cells result in considerableattention being directed to improving the efficiency and completeness ofthe power generating electrochemical reactions which occur therein, aswell as to increasing the amount of reactive material which can beplaced into the cell.

In general, metal-air cells convert atmospheric oxygen to hydroxyl ionsin the air cathode of the cell. The hydroxyl ions then migrate to theanode, where they cause the metal contained in the anode to oxidize.Usually the active anode material in such cells comprises zinc.

More particularly, the desired reaction at a metal-air cell air cathodeinvolves the reduction of oxygen, the consumption of electrons, and theproduction of hydroxyl ions, the hydroxyl ions being free to thenmigrate through the electrolyte toward the anode, where oxidation ofzinc may occur, forming zinc oxide.

In most metal-air cells, air enters the cell through one or more portsin the cell. The ports extend through the bottom of the cathode can, andmay be immediately adjacent the cathode assembly, or may be separatedfrom the cathode assembly by an air chamber or an air diffusion member.

In any such arrangements, the port facilitates the movement of air intothe cathode assembly. At the cathode assembly, the oxygen in the airreacts with water, as a chemically reactive participant in theelectrochemical reaction of the cell, and thereby forms the hydroxylions.

In order for the electrochemical cell to survive normal conditionsencountered in fabrication and use of the cell, the respectivestructural components of the cell must be able to withstand the normalconditions of fabrication and use, both individually and in combination.The main structural components are the anode can and the cathode can,plus an intervening seal which provides support between the anode canand the cathode can.

In general, the size of any given cell is limited by the insidedimensions of the space provided in the article in which the cell willoperate. For example, the size of a hearing aid cell is limited to theinternal dimensions of the cavity in the hearing aid appliance. Theinternal dimensions of the cavity are determined by the hearing aidmanufacturer, not the power cell manufacturer.

Thus, any given appliance includes a limited amount of grossspace/volume allotted to occupancy by the electrochemical cell whichpowers the appliance. That gross space is ultimately divided accordingto three competing, but supportive functions. A first and minimalportion of the gross space is used to provide clearance between theinterior elements of the space and the exterior elements of theelectrochemical cell. A second portion of the gross space is occupied bythe structural and otherwise non-reactive elements of theelectrochemical cell. The third portion of the gross space is occupiedby the electrochemically reactive materials of the electrochemical cell.Since

Since the overall electrochemical capacity of any electrochemical cellis to some extent determined by the quantity of electrochemicallyreactive materials which can be loaded into the cell, it is important tomaximize the volume of the space devoted to containing the reactivematerials. It is correspondingly important to minimize the portions ofthe space that are used for providing clearance for the cell, and forproviding structural support and other non-reactive elements within thecell.

Normal conditions of fabrication of the cell place significantstructural stresses on both the anode can and the cathode can.Specifically, when the anode can and the cathode can are assembled toeach other, a force pushing the cathode can toward the anode can may beused to correspondingly crimp the distal edge of the cathode cansidewall against the anode can and the intervening seal. An opposingforce may be exerted on the anode can as part of the assembly process.

The physical properties of the structural elements of the is cell,namely the anode can and the cathode can, must be strong enough towithstand especially the opposing forces used in assembling and closingthe cell. Thus, the respective enclosing top, bottom, and side walls ofthe anode can and the cathode can must be strong enough to tolerate theassembly process without collapsing. Using conventional can structuresand closure processes has, prior to this invention, suggested that thethickness of the can side walls be of the order of at least 0.008 inchin order for the cans to predictably tolerate the assembly process. If,however, the thickness of the side walls could be reduced, that wouldrelease additional internal volume in the cell for use in holding theelectrochemically reactive material, e.g. metal anode material.

A further problem experienced with electrochemical cells of the"button-type" construction is that the bottom of the cathode can tendsto become dished-in/concave during closing of the cell at finalassembly. This problem is related to thickness of the metal used forfabricating the cathode can, and becomes more acute as one reduces thethickness of the metal.

It is an object of this invention to provide improved electrode canstructure, especially cathode can structure, for an electrochemical cellby providing improved structure of the can at the corner joining thebottom of the can to a corresponding sidewall of the can.

It is another object to provide improved cathode can structure for ametal-air electrochemical cell, by providing improved structure of thecan at the corner joining the bottom of the can to a corresponding sidewall of the can.

It is yet another object to provide an electrode can having a bottom,and a side wall extending upwardly from the bottom, wherein force can beapplied upwardly on the flat surface of the bottom of the can, and canbe transmitted away from the bottom in a direct line through a side wallof the can and parallel to an inner surface of the side wall.

It is yet another object to provide an electrochemical cell wherein thecathode can, the anode can, or both, include improved structure of therespective can at the corner joining the closed end of the respectivecan to side wall of the respective can.

It is still another object to provide improved methods for forming thecorner between the closed end of the respective can and thecorresponding side wall of the can.

It is a further object to provide improved methods for assembling ananode, including an anode can, to a cathode, including a cathode can, tothus make a button-type electrochemical cell, including closing the cellso made by crimping the distal edge of the cathode side wall against theanode can and an intervening seal, while exerting a limited opposingforce on the anode can.

SUMMARY OF THE DISCLOSURE

Some of the objects are obtained in a first family of embodimentscomprehending an electrode can for use in an electrochemical cell, theelectrode can comprising a bottom, having a first inner surface, and afirst outer surface having a substantially flat portion extendingradially outwardly to a first outer perimeter of the first outersurface; and at least one side wall extending upwardly from the bottom,the at least one side wall having a second outer surface and a secondinner surface, the substantially flat portion of the first outer surfaceof the bottom extending radially outwardly of the second inner surfaceof the at least one side wall.

In preferred ones of the embodiments, the outer perimeter of the firstouter surface of the bottom is substantially confined radially outwardlyof the second inner surface of the at least one side wall.

Preferred embodiments further include an intermediate element of the canextending between the bottom and the at least one side wall, theintermediate element comprising a curvilinear third outer surfaceextending between the outer perimeter of the first outer surface of thebottom and the second outer surface of the at least one side wall, thecurvilinear third outer surface being substantially confined radiallyoutwardly of the second inner surface of the at least one side wall.

Preferably, the substantially flat portion covers substantially theentirety of the first outer surface of the bottom.

Where the electrode can includes the intermediate element, the bottommay further comprise a first inner perimeter of the first inner surface,and the intermediate element comprises a curvilinear third inner surfaceextending between the first inner perimeter of the first inner surfaceand the second inner surface of the at least one side wall, thecurvilinear third inner surface describing a curvature having an averageradius of less than 0.125 mm, preferably no more than 0.050 mm, morepreferably no more than 0.025 mm. Most preferably, the third innersurface represents a sharp corner at the joinder of the first and secondinner surfaces.

Preferred average thickness for the at least one side wall is no greaterthan 0.175 mm, preferably no greater than 0.125 mm, the bottompreferably having corresponding thickness.

Preferred application for the electrode can of the invention is as acathode can, although use as an anode can is also contemplated.

Where the intermediate element is used, the intermediate element istypically work-hardened as respects the bottom and the at least one sidewall.

In a second family of embodiments, the invention comprehends anelectrode can for use in an electrochemical cell, the electrode cancomprising a bottom, having a first inner surface, and a first outersurface having a substantially flat portion extending radially outwardlyto a first outer perimeter of the first outer surface; and at least oneside wall extending upwardly; away from the bottom, the at least oneside wall having a second outer surface and a second inner surface, thebottom and the at least one side wall being juxtaposed with respect toeach other such that a force directed against the substantially flatportion of the first outer surface of the bottom can be transmitted awayfrom the bottom in a straight line extending through the at least oneside wall and parallel to the second inner surface.

In a third family of embodiments, the invention comprehends anelectrochemical cell, comprising an anode, the anode comprising an anodecan and electrochemically reactive anode material; a cathode, thecathode comprising a cathode can and a cathode assembly inside thecathode can, the cathode can comprising (i) a bottom, having a firstinner surface, and a first outer surface having a substantially flatportion extending radially outwardly to a first outer perimeter of thefirst outer surface, and (ii) at least one side wall extending upwardlyfrom the bottom, the at least one side wall having a second outersurface and a second inner surface, the substantially flat portion ofthe first outer surface of the bottom extending radially outwardly ofthe second inner surface of the at least one side wall; and anelectrically insulating seal between the anode and the cathode.

In preferred ones of the embodiments of the electrochemical cell, theouter perimeter of the first outer surface of the bottom issubstantially confined radially outwardly of the second inner surface ofthe at least one side wall.

Preferred electrochemical cells include an intermediate element of thecathode can extending between the bottom and the at least one side wall,the intermediate element comprising a curvilinear third outer surfaceextending between the outer perimeter of the first outer surface of thebottom and the second outer surface of the at least one side wall, thecurvilinear third outer surface being substantially confined radiallyoutwardly of the second inner surface of the at least one side wall.

In the electrochemical cell, the substantially flat portion of thebottom of the cathode can preferably covers substantially the entiretyof the first outer surface of the bottom.

The side wall of the cathode can preferably has an average thickness nogreater than 0.175 mm, preferably no greater than 0.125 mm, the bottomof the cathode can preferably having corresponding thickness.

Where the intermediate element is used in the cathode can, theintermediate element is typically work-hardened with respect to thebottom of the cathode can and with respect to the at least one side wallof the cathode can.

In a fourth family of embodiments, the invention comprehends a method ofmaking an electrochemical cell comprising an open-ended anode, receivedin an open-ended cathode, and an electrically insulating seal betweenadjacent elements of the anode and the cathode, the anode including ananode can and reactive anode material, the cathode including a cathodecan and a cathode assembly inside the cathode can. The method comprisesthe steps of fabricating a cathode can having (i) a bottom, including afirst inner surface, and a first outer surface having a substantiallyflat portion extending radially outwardly to a first outer perimeter ofthe first outer surface, and (ii) at least one side wall extendingupwardly from the bottom to a distal edge thereof, the at least one sidewall having a second outer surface and a second inner surface, the firstouter perimeter of the first outer surface of the bottom extendingradially outwardly of the second inner surface of the at least one sidewall; placing a cathode subassembly in the cathode can formed in theprevious step, to make the cathode; selecting a suitable anode for usewith the cathode made in the previous step; placing the anode andcathode in juxtaposed relation to each other, with the seal memberdisposed in position for being assembled between adjacent elements ofthe anode and the cathode; and urging the cathode toward the anode andagainst a crimping die element with a first force sufficient to bringthe anode into assembled relation inside the cathode can, with the sealbetween adjacent elements of the anode and the cathode, and toconcurrently crimp the distal edges of the at least one side wall of thecathode can, through use of the crimping die element, against the sealand against the anode, to thereby join the cathode to the anode inassembling the electrochemical cell; and while urging the cathodeagainst the crimping die element and crimping the distal edges of thecathode can against the seal and the anode, urging the anode can towardthe cathode with a force of no more than 135 pounds.

Preferably, the force used in urging the anode can toward the cathode isno more than 70 pounds, more preferably no more than 45 pounds, mostpreferably no more than about 30 pounds.

In a fourth family of embodiments, the invention comprehends a method ofmaking an electrode can for use in a metal-air electrochemical cell, theelectrode can having a bottom, at least one side wall extending upwardlyfrom the bottom, and an intermediate element extending between thebottom and the at least one side wall, the bottom having a first innersurface and a first outer surface, the first outer surface having asubstantially flat portion extending radially outwardly to a finishedouter perimeter of the first outer surface, the at least one side wallhaving a second inner surface defining a finished inner perimeter and asecond outer surface defining a finished outer perimeter, the electrodecan having a finished depth, the substantially flat portion of thebottom extending radially outwardly of the second inner surface of theat least one side wall. The method comprises the steps of fabricating ametal can pre-form, the can pre-form having an inside surface and anoutside surface, defined by a pre-form bottom, and at least one pre-formside wall extending upwardly from the pre-form bottom, the pre-formbottom having a first pre-form inner surface, a pre-form first outersurface, and a pre-form first outer perimeter, the at least one pre-formside wall having a pre-form second inner surface and a pre-form secondouter surface, a pre-form intermediate element extending between thepre-form bottom and the at least one pre-form side wall, the at leastone pre-form sidewall comprising a pre-form inner perimeter extendingabout the pre-form first inner surface thereof, and a pre-form secondouter perimeter extending about the pre-form second outer surfacethereof, the pre-form can having a pre-form depth, the pre-formintermediate element comprising a pre-form curvilinear third outersurface extending between the pre-form second outer surface of the atleast one pre-form side wall and the pre-form first outer perimeter ofthe pre-form bottom, a pre-form curvilinear third inner surfaceextending between the pre-form second inner surface of the at least onepre-form side wall and the pre-form first inner surface of the pre-formbottom, the pre-form curvilinear third outer surface extending inwardlyof the pre-form second inner surface, such that a force directed againstthe bottom of the pre-form can can be transmitted away from the bottom,and into the pre-form at least one side wall in a straight line onlywhere the line is transverse to the pre-form second inner surface of thepre-form at least one side wall; and press-forming the pre-form can sofabricated in a pressing step to make the finished cathode can havingthe finished dimensions, comprising (i) forming the can pre-form on theinside surface thereof with a first die element of a die set, the firstdie element comprising a small radius edge disposed against the pre-formcurvilinear third inner surface to thereby reduce the pre-formcurvilinear third inner surface to an average radius of no more than 50microns, and (ii) concurrently forming the can pre-form on the outsidesurface thereof with a second die element of the die set, to re-shapethe pre-form curvilinear third outer surface, and to thereby make thecurvilinear third outer surface to be disposed substantially entirelyoutside the finished inner perimeter of the at least one side wall, suchthat a force directed against the bottom of the can can be transmittedaway from the bottom, and into the at least one side wall, in a straightline parallel to the second inner surface of the at least one side wall.

It is preferred that fabricating the cathode can from the pre-form tothe finished dimensions include reducing the pre-form curvilinear thirdinner surface to an average radius of no more than 0.025 mm.

In generally preferred embodiments, the method includes press formingthe pre-formed workpiece such that the first and second elements of thedie set are concurrently in intimate relationship with substantially theentirety of opposing inner and outer surfaces of the workpiece. Themethod typically results in a pre-form can wherein a force directedperpendicular to the bottom of the pre-form can imparts a net bendingmoment to the bottom of the pre-form can, with respect to the pre-format least one side wall, and wherein a corresponding force directedperpendicular to the bottom of the finished cathode can is transferredto the at least one side wall without imparting a net bending moment tothe bottom of the can.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross section of an assembly of a cathode can, an anodecan, and a seal, illustrating the concavity typical of prior art cathodecans.

FIG. 1A is an enlarged fragmentary cross-section, showing the cornerstructure of the prior art cathode can illustrated in FIG. 1.

FIG. 2 shows a cross section of an electrochemical cell of theinvention.

FIG. 2A is an enlarged fragmentary cross-section, showing the idealcorner structure of the cathode can of the invention illustrated in FIG.2.

FIG. 2B is an enlarged fragmentary cross-section, showing the cornerstructure of the cathode can of the invention illustrated in FIG. 2under actual conditions.

FIG. 3 illustrates the die arrangement, and the final stage of themethod used to bring together the anode, the cathode, and the seal, tothereby close and crimp the respective elements in assembly of theelectrochemical cell.

FIG. 4 shows a pictorial view, with parts rotated for visibility, of thecathode assembly used in the electrochemical cell shown in FIG. 2.

FIG. 5 shows an enlarged fragmentary cross-section of the cathodeassembly of FIG. 4, taken at 5--5 of FIG. 4.

FIGS. 6A-6C illustrate stages of fabrication of the electrode can of theinvention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

FIGS. 1 and 1A show a cross section of a prior art assembly of a cathodecan, an anode can, and a seal. The dashed line in FIG. 1 represents astraight line across the cathode can between elements of the outerperimeter of outer surface of the bottom of the cathode can, whichstraight line would be coincident with a flat bottom on the cathode can.The solid line next above the dashed line represents the actual outersurface of the bottom of the cathode can. FIG. 1 illustrates thephenomenon common in cells of the prior art, wherein the bottom of thecathode can is permanently displaced upwardly by the force used injoining the cathode and the anode at final assembly of the cell. Suchdisplacement, of course, reduces the usable volume inside the cell, withcorresponding reduction in total cell discharge capacity.

FIGS. 2 and 2A generally represent the improved electrode cans, andcorresponding electrochemical cells, provided by the invention. As shownin FIGS. 2 and 2A, the cell can be closed, and the cathode can crimpedabout the anode can at final assembly, while maintaining flat the outersurface of the bottom of the cathode can.

Referring now by characters of reference, FIGS. 2 and 2A generallyrepresent a cross-section of a metal-air cell 10, preferably a zinc-aircell, of the present invention. The negative electrode 12, also referredto as the anode, is electrically insulated from the positive electrode14, also referred to as the cathode, by a nylon or similar seal 16.

Cathode 14 is generally comprised of a cathode assembly 18, containedwithin cathode can 20. Cathode can 20 has a bottom 22, andcircumferential upstanding side wall 24 extending upwardly from thebottom.

The bottom 22 typically has an outer surface 26. At least a portion ofthe outer surface 26 is substantially flat, the substantially flatportion extending to an outer perimeter 27, best seen in FIG. 2A.Preferably, the entirety of the outer surface 26 is flat. Bottom 22further has a generally flat inner surface 28, disposed opposite outersurface 26. Similarly, the side wall 24 has inner surface 30 and outersurface 32, the inner and outer surfaces 30, 32 extending about thecircumference of the can, and defining the side wall therebetween.

The side wall 24 is joined to the bottom 22 of the can by intermediateelement 34. See FIG. 2A. The outer surface 35 of intermediate element 34extends, from its lower end at outer perimeter 27 of outer surface 26 ofbottom 22, to its upper end 36 which joins the outer surface 32 of theside wall 24 in a generally vertical orientation. The inner surface 37,if any, of the intermediate element 34 is represented at the joinder ofthe inner surface 28 of the bottom 22 and the inner surface 30 of theside wall 24. In preferred embodiments of the invention, the innersurfaces 28 and 30 come together at a sharp corner, such that the innersurface 37 is of nominal dimension. To the extent the corner material isworked in forming the corner, the corner is work hardened, whereby thecorner structure is strengthened with respect to bottom 22 and side wall24 as the corner structure is formed at intermediate element 34.

Whereas in the prior art, the inner surface corresponding to 37describes a radius of e.g. 0.005 inch (0.127 mm), the inner surface 37of electrode cans of the invention describe a radius of less than 0.125mm. A radius of 0.075 mm or less is an improvement over the prior art. Aradius of 0.050 mm is good, with 0.025 mm being even better. Preferablythe coming together of the inner surfaces 28 and 30 describes a sharpinterior corner at 37, whereupon the representation of intermediatesection 34 is nominal at locus 37. A "sharp interior corner," means thatthe radius is substantially zero, or as close to zero as can be madewith standard machine tools.

As illustrated in FIGS. 2 and 2A, in the illustrated embodiment, theinner surface 30 of the side wall 24 extends generally perpendicular tothe outer surface 26 of bottom 22 of the cathode can. As seen therein, astraight line extension of inner surface 30 intersects the outer surface26 of the bottom 22 at a locus 29 disposed inwardly of the outerperimeter 27. Correspondingly, the outer perimeter 27 extends radiallyoutwardly of the inner surface 30 of the side wall 24.

The dimension between the opposing arrows 38 (FIG. 2A) defines a forcetransmitting portion 40 of the bottom 22 extending from locus 29 toouter perimeter 27, and upwardly from the outer surface 26 toward theside wall 24, and which transmitting portion 40 can transmit upwardlydirected forces from the outer surface 26 to the side wall 24 in astraight line parallel to the inner surface 30 of side wall 24. Thedouble headed arrow in FIG. 2A illustrates a force 42 applied uniformlyacross the outer surface 26, including at transmitting portion 40, whichforce is transmitted to the side wall 24 through the force transmittingportion 40 of the bottom 22 between the outer perimeter 27 and the locus29, locus 29 being projected from inner surface 30 of side wall 24.

As upwardly directed force 42 is applied to bottom 22 at its outersurface 26, the force is transmitted upwardly in a straight line throughthe transmitting portion 40 defined by the area defined between opposingarrows 38. As illustrated by the double headed arrow at 42, and thecorresponding dashed line 44 extending upwardly from the arrow at 42,such force applies no effective bending moment to the bottom surface 26with respect to the side wall 24, because there is no effective leverarm between the point where the force is applied at transmitting portion40 and the side wall 24. Rather, the force travels in a straight linedirection indicated by the dashed line 44, which is aligned with thedirection of application of the force from the outer surface 26, intothe side wall 24.

By contrast, and now referring to FIG. 1A, the inventor herein, bycareful inspection, discovered that, in the prior art structure of thecathode can, the outer perimeter of the flat outer surface of the bottomwas disposed inwardly of the projected inner surface of thecorresponding side wall, the side wall being perpendicular to the bottomwall.

The inventor herein further discovered that a vertically directed forceapplied to the bottom of the can, even at its outer surface, such as theforce applied to the cathode in joining the anode and cathode andcrimping the cathode about the anode and seal, was being transmittedupwardly into the side wall along lines extending (i) transverse to theinner surface of the side wall, and (ii) transverse to the direction ofapplication of the vertical force. The corresponding upwardly directedforce on the outer surface of the bottom of the cathode can isillustrated in FIG. 1A by the double headed arrow at the outer perimeterof the can bottom. A straight transverse line transmitting that forceinto the side wall is indicated by the transverse dashed line extendingfrom the double headed arrow at the outer surface of the bottom, (at theouter perimeter) through the corresponding intermediate element and intothe side wall. The horizontal component of the line of force representsa lever arm between the bottom of the can and the side wall. Thus, anupwardly directed force on the bottom of the prior art can, as indicatedby the double headed arrow in FIG. 1A, exerts a bending moment on thebottom of the can.

Conventional physics analysis teaches that the bending moment is theproduct of (a) the magnitude of the force applied and (b) the lever armover which the force is applied. The lever arm is illustrated in FIG. 1Aby the opposing arrows at the bottom of the can. Thus, as suggested bythe parallel dashed lines along the bottom of the can in FIG. 1A, whenforce is applied against the bottom of a cathode can of the prior art,the bending moment urges the bottom of the can to deform upwardly.Where, as in closure of the cell, and crimping of the cathode caninwardly on the seal and the anode can, the upwardly directed force isgreat enough, the bottom of the cathode can may be permanentlydeformed/displaced by the upwardly directed force, as shown in FIGS. 1and 1A.

FIG. 2B shows the cathode can of the invention as it is often formedunder actual conditions, wherein intermediate element 34 bulgesoutwardly, and locus 37 forms a slight undercut in inner surface 30.

FIG. 3 illustrates the die arrangement, and the final stage of theprocess of bringing together the anode, the cathode, and the interveningseal, to close the cell and crimp the distal edges of the cathode sidewall about the anode. Referring now specifically to FIG. 3, a closingpunch 46 exerts an upwardly directed force 42, indicated by the doubleheaded arrows, against the bottom 22 of the cathode can 20. An opposingejector punch 48 exerts an opposing downwardly directed force 50 oflesser magnitude, indicated by the double headed arrows, against the top52 of the anode can 54. Closing die 56 generally surrounds the cavity 58in which the cell is closed and crimped.

As the closing punch 46 exerts the force 42 indicated by arrows, thusurging the cathode can against the anode can and the seal, the cathodecan is moved into the closing die 56 and against the transverse crimpinglands 60 of the die. At the same time, the ejector punch 48 exerts itsopposing force 50 against the anode. In order for the closing punch 46to force the cathode can closed, it must exert sufficient force to formthe crimp on the distal edges 62 of the side wall 24, thus grippinganode can 54 and seal 16 in closure of the cell 10, as well assufficient force to overcome the resistance of the opposing force 50 onthe anode can.

The force 50 on the anode can has at least two purposes. First, theforce 50 on the anode can helps stabilize the anode can while the cellis being closed. Second, and referring to FIG. 1A, the force 50 isexerted through seal 16 against the inner surface 28 of the bottom 22 ofthe cathode can, and thus generally opposes any bending moment which maybe imposed on the bottom 22 of the cathode can, thus tending to reducethe deformation suggested in FIG. 1A. The magnitude of the force 50 isgenerally determined with respect to the need to oppose the bendingmoment, as the bulk of the force 42 on the cathode can can be absorbedby the stationary closing dies 56. Thus, to the extent the need tooppose bending moment on the cathode can is reduced, the magnituderequired of the force 50 on the anode can is correspondingly reduced.

The effectiveness of the force 50 in attenuating the tendency of thebottom of the cathode can to deform is related to the magnitudes of therespective forces 42 and 50, the lever arms over which they are appliedrelative to the respective can side walls, and the thickness of thematerial making up bottom 22. To the extent the thickness of thematerial is reduced, e.g. to correspondingly increase usable interiorvolume, the contribution of the metal thickness to resisting bendingforces is similarly reduced. Further, the lever arm between the anodecan side wall and the cathode can side wall is also reduced when canwall thickness is reduced.

In order for the opposing force 50 to remain effective in stabilizingthe bottom 22 as the cell is closed, the magnitude of the force 50 mustbe increased to offset any reduced contributions of the lever arm andthe bottom thickness. But as the thickness of the side wall e.g. 63 ofthe anode can is reduced, the capacity of the side wall 63 to toleratethe downward force 50 is reduced. Indeed, the normal 500 lbs of forceconventionally used with an anode side wall thickness of 0.008 inch(0.203 mm) can cause the anode can to collapse during the closingprocess where the thickness of the side wall is reduced to less than0.008 inch (0.203 mm).

Accordingly, if the thickness of the side walls 24, 63 are to bereduced, the magnitude of the force 50 must be reduced, whereby someother mechanism must be provided for controlling or eliminating thetendency of the bottom of the cathode can to deform upwardly.

Referring again to FIG. 2A, it is noted that the force transmittingportion 40 is displaced outwardly from the corresponding locus whereforce is transmitted from the bottom surface of the cathode can in FIG.1A. Thus, force 42 in FIG. 2A is transmitted from the outer surface 26of the bottom of the cathode can in a straight line upwardly to the sidewall, without imposing any bending moment on the bottom 22. The onlybending moment, if any, on the bottom 22 is that corresponding withtransferring, to the transmitting portion 40, any force applied to thecan inside the inner perimeter of the force transmitting portion 40 e.g.inside the locus defined at 29. Such bending moment, if any, isconsiderably less than the bending moment applied in FIG. 1A, where allforce applied produces a bending moment.

Since force 42 applies little or no effective bending moment on bottom22, little or no opposing force 50 is needed to correct a bending momentfrom force 42. A direct result of the outward displacement of the forcetransmitting portion 40 is thus a reduction in the required magnitude ofthe opposing force 50, from a magnitude of about 500 lbs using cans ofthe prior art to a magnitude of about 70 lbs or less, e.g. usingotherwise similar cans of the invention. Accordingly, metal stripsuitable for forming cathode cans of the invention for use in 675A cellsmay be preferably about 0.150 mm to about 0.180 mm thick, morepreferably about 0.160 mm to about 0.170 mm thick, most preferably about0.165 mm thick. Metal strip suitable for forming cathode cans of theinvention for use in size 13A cells may be preferably about 0.110 mm toabout 0.130 mm thick, more preferably about 0.115 mm to about 0.125 mmthick, most preferably about 0.119 mm thick.

The retention of the flat configuration at the bottom of the canprovides a corresponding increase in the useful volume inside the cell,which translates almost completely to an increase in space which can befilled with additional reactive material.

Returning now to further general description of the cells of theinvention, the bottom 22 of the cathode can 20 is perforatedtherethrough by air ingress holes, also known as air ports 64. Thecathode assembly 18 is interposed between the anode 12 and the bottom 22of the cathode can. Anode 12 is comprised of a mixture of zinc powderand electrolyte, the composition of the electrolyte comprising about 2%by weight zinc oxide, 68% by weight water, and 30% by weight potassiumhydroxide.

FIG. 4 shows a perspective view of the cathode assembly used in cellsrepresentative of the present invention. Active layer 66 is interposedbetween barrier layer 68 and air diffusion layer 70. Active layer 66ranges preferably between about 0.002 and about 0.05 inch thick, andfacilitates the reaction between the hydroxyl in the electrolyte and thecathodic oxygen of the air. Barrier layer 68 is a micro-porous plasticmembrane about 0.001 inch thick, typically polypropylene, having theprimary function of preventing anodic zinc particles from coming intophysical contact with the remaining elements of the cathode assembly 18.Barrier layer 68 however, does permit passage of hydroxyl ions and watertherethrough. Air diffusion layer 70 is preferably a micro-poroushydrophobic polymeric material such as a polytetrafluoroethylene (PTFE)membrane about 0.004 inch thick, which permits passage of airtherethrough. The air diffusion layer 70 may be used to limit thecurrent density produced by the cell to a desired maximum. Air diffusionlayer 70 is further impervious to battery electrolyte.

FIG. 5 is an enlarged perspective view of the cathode assembly 18.Active layer 66 is further comprised of connecting substratum capable ofbeing connected to electrical circuitry, namely conductive woven nickelwire layer 72. Carbon, indicated at 74, preferably forms a matrixsurrounding the conductive layer 72 of nickel wire. Nickel is preferredfor layer 72 because nickel exhibits little or no corrosion in thealkaline environment of the zinc-air cell, and also because nickel is anexcellent electrical conductor.

The electrode cans (e.g. cathode can 20) of the invention can befabricated using a conventional sheet metal press. For example, acathode can for a size 13A zinc air cell was made as follows.

EXAMPLE

Nickel plated steel 0.008 inch thick was obtained from Hille & Muller,Dusseldorf, Germany. The metal was fabricated into cathode cans using a45 Ton press having three draw stations. Prior to fabrication in thepress, the metal was lubricated with forming lubricant. At Draw StationNo. 1, a cup representing a rough approximation of the finished can wasstamped in the metal strip. At Draw Station No. 2, the cup was furtherworked, to more closely approach the dimensions of the finished can. AtDraw Station No. 3, the final can dimensions were generated. After DrawStation No. 3, the workpiece/cup was separated from the metal strip andtrimmed, to complete fabrication of the cathode can.

FIGS. 6A-6C illustrate the progressive formation of the workpiece at thethree draw stations. Following Table 1 illustrates the conditions of theworkpiece at the conclusion of operation at each draw station.

"O/Dia" means outside diameter of the workpiece.

"I/Dial" means inside diameter of the workpiece.

"Depth" refers to the inside depth of the workpiece.

"Radius" refers to the radius described by the surface corresponding toinner surface 37 of the intermediate element 34 at the respective drawstations.

                  TABLE 1                                                         ______________________________________                                        Station No.                                                                            O/Dia     I/Dia     Depth   Radius                                   ______________________________________                                        1 (FIG. 6A)                                                                            9.80 mm   9.40 mm   3.76 mm .800 mm                                  2 (FIG. 6B)                                                                            8.54 mm   8.14 mm   4.82 mm .400 mm                                  3 (FIG. 6C)                                                                            7.75 mm   7.35 mm   5.43 mm Sharp                                    ______________________________________                                    

A suitable cathode assembly 18 was placed in the cathode can. A suitableanode, including anode can and anode material was selected. The anode, asuitable seal 16, and the cathode were placed in juxtaposed positionwith respect to each other and the anode and seal were urged part-wayinto the cathode can, to thus partially close the cell, and prepare itfor final closure and sealing.

The partially closed cell was then placed in a closing assemblyincluding a closing punch 46, closing die 56, and ejector punch 48, allas illustrated in FIG. 3. Closing die 56 then applied closing force 42against the outer surface 26 of the bottom 22 of the cathode can whileejector punch 48 applied opposing force 50 of lesser magnitude. Underthe influence of forces 42 and 50, the cathode, the anode, and the sealmoved together into the closing die 56 and the cathode was fully seatedover the anode. As the cathode, the anode, and the seal moved into theclosing die, the distal edge 62 of cathode side wall 24 was engaged bythe crimping lands 60 of the die 56, crimping the edge 62 inwardlyagainst seal 16 and anode can 54 and thus closing and sealing the cell10. By the time the cell had been so closed, the force 42 applied by theclosing punch 46 reached about 1000 lbs, and the opposing force 50reached about 70 lbs. The resulting cell was structurally sound. Thebottom was not dished. All components met desired dimensionspecifications.

Given the reduced force 50 on the anode can, the structural requirementsfor tolerating compressive stress on the can are reduced. As a result,thinner metal can be used for fabricating the can 10, whether cathodecan or anode can, while meeting the reduced stress requirements. Forexample, for size 675A cells, the metal can be reduced at least to0.0065 inch (0.165 mm), and for size 13A cells, the metal can be reducedat least to 0.0047 inch (0.119 mm).

The above structural improvements can be practiced, making improved cansusing a variety of metal structures. Plating materials and ductility arethe important characteristics of the electrode can. The can may beformed of virtually any metal that is plated or clad with theappropriate metal, such appropriate metal having a hydrogen overvoltagesimilar to that of the corresponding electrode and being insoluble athigh pH's (or in the presence of electrolyte), the metal plating orcladding being in chemical communication via the electrolyte with theelectrode material, if not in direct physical contact therewith.

Optionally, the can may be formed entirely of a metal or alloy having ahydrogen overvoltage similar to that of the electrode (as opposed toplating or cladding the can). In addition to nickel, stainless steel,palladium, silver, platinum, and gold may be suitable plating, cladding,or can materials. Steel strip plated with nickel and nickel alloy isgenerally used because the cost is low, and because pre-plated steelstrip, which generally requires no post-plating processes, iscommercially available. The metal in the can must be ductile enough towithstand the drawing process, and strong enough to withstand the cellcrimping and closure process.

Cathode cans, for example, may be made of cold-rolled steel plated withnickel. Steel strip pre-plated with nickel can also be used. Cathodecans may also be formed from cold-rolled mild steel, with at least theinside portions of the cans being subsequently post plated with nickel.Other specific examples of materials for cathode cans includenickel-clad stainless steel; nickel-plated stainless steel; INCONEL(INCO alloy of nickel, a non-magnetic alloy); pure nickel with minoralloying elements (NICKEL 200 and related family of NICKEL 200 alloyssuch as NICKEL 201, etc.), all available from Huntington Alloys, adivision of INCO, Huntington, W. Va. Some noble metals may also find useas plating, cladding, or can metals, including steel strip plated withnickel, and mild steel strip subsequently plated with nickel afterforming the can.

As illustrated above, the can is readily made using a press having threedraw stations. The can is also readily made using a press having twostations. Other methods of forming the can, having the inventiveconfiguration shown herein, will be readily apparent to those skilled inthe art.

The process described above for making a cathode can is also useful formaking corresponding anode cans having related improvements in physicalproperties. Such making of anode cans would, of course, be practicedusing metal structures compatible with the polarity of the anode. Forexample, an anode can is preferably plated with copper on its innersurface. Copper has a hydrogen overvoltage similar to that of zinc. Ananode can is readily formed of stainless steel wherein the inner surfaceis plated with copper, and the outer surface is plated with nickel. Suchcan is available with a prior art conventional intermediate elementstructure as in FIG. 1A from, for example, Hitachi, Japan.

Thus, the invention contemplates making an electrochemical cell using acathode can having the outwardly displaced force transmitting portion40, in combination with an anode can having a correspondingly outwardlydisplaced force transmitting portion on the closed top of the anode can.The invention also contemplates, as illustrated in FIGS. 2 and 2A, usinga cathode can of the invention in combination with an anode can of theprior art, wherein the conventional corner radius defines the forcetransmitting portion inwardly of the inner side wall of the anode can.

Those skilled in the art will now see that certain modifications can bemade to the articles, apparatus, and methods herein disclosed withrespect to the illustrated embodiments, without departing from thespirit of the instant invention. And while the invention has beendescribed above with respect to the preferred embodiments, it will beunderstood that the invention is adapted to numerous rearrangements,modifications, and alterations, and all such arrangements,modifications, and alterations are intended to be within the scope ofthe appended claims.

Having thus described the invention, what is claimed is:
 1. An electrodecan for use in an electrochemical cell, said electrode cancomprising:(a) a bottom, having a first inner surface, and a first outersurface; (b) at least one side wall extending upwardly, away from saidbottom, said at least one side wall having a second outer surface and asecond inner surface, said second inner surface extending substantiallyperpendicular to the first outer surface such that a straight lineextension of said second inner surface intersects said first outersurface at a locus disposed inwardly of a first outer surface perimeter;and (c) an intermediate element of said can extending between saidbottom and said at least one side wall, said intermediate elementcomprising a third outer surface and a third inner surface, said thirdouter surface extending between said outer perimeter of said first outersurface of said bottom and said second outer surface of said at leastone side wall, said third inner surface extending between said firstinner perimeter of said first inner surface and said second innersurface of said at least one side wall, said third outer surface bulgingoutwardly from said at least one side wall,said electrode can havingbeen fabricated prior to incorporation of said electrode can into anelectrochemical cell.
 2. An electrode can as in claim 1, said thirdinner surface having an undercut with respect to, and extendingoutwardly of, said second inner surface.
 3. An electrode can as in claim2, said third inner surface describing a curvature between said firstand second inner surfaces having an average radius of less than 0.125mm.
 4. An eletrode can as in claim 1, said at least one side wall havingan average thickness from about 0.110 mm to about 0.130 mm.
 5. Anelectrode can as in claim 1, said at least one side wall having anaverage thickness no greater than 0.125 mm.
 6. An electrode can as inclaim 1, said at least one side wall having an average thickness fromabout 0.115 mm to about 0.125 mm.
 7. An electrode can as in claim 1,said at least one side wall having an average thickness of about 0.119mm.
 8. An electrode can as in claim 1 wherein said intermediate elementis work-hardened with respect to said bottom and said at least one sidewall.
 9. An electrode can as in claim 1, said third inner surfacedescribing a curvature between said first and second inner surfaceshaving an average radius of less than 0.075 mm.
 10. An electrode can asin claim 1, said third inner surface describing a curvature between saidfirst and second inner surfaces having an average radius of less than0.050 mm.
 11. An electrode can as in claim 1, said third inner surfacedescribing a curvature between said first and second inner surfaceshaving an average radius of less than 0.025 mm.
 12. An electrode can asin claim 7, said third inner surface describing a sharp interior cornerwith respect to said second inner surface of said at least one sidewall.